In the manufacture of a tire, the tire is typically built on the respective drum of one or more tire-building machines, which is known in the art as a tire building drum. Numerous tire components are wrapped about and/or applied to the drum(s) in sequence, forming a cylindrical shaped tire carcass. The tire carcass is then expanded into a toroidal shape for receipt of the remaining components of the tire such as the belt package and a rubber tread. The completed toroidally-shaped unvulcanized tire carcass, which is known in the art as a green tire, is then inserted into a mold or press for forming of the tread pattern and curing or vulcanization.
When components are wrapped about and/or applied to the tire building drum(s), typically the inner liner, one or more plies and possibly other components are wrapped onto the drum while the drum is in a collapsed state, before tire beads are applied. The beads are then located axially on the drum and the areas of the drum under the beads, known as bead locks, are expanded radially to fix the rest of the tire to the beads. The beads are then moved closer together by axial contraction of the drum, while the area of the drum between the beads, known as the center mechanism, is expanded to form a shoulder to help anchor the beads and provide a surface to apply subsequent components. Such expansion of the center mechanism of the tire building drum is referred to as crowning.
It is desirable that a crown, which is the radial difference between the expanded diameter of the center mechanism and the nominal tire bead diameter, be sufficiently large to enable subsequent components to be applied near their final shape and size. A large crown minimizes distortion when the tire is shaped during the remaining tire building and curing steps. For example, it is desirable to position apex strips vertically against each crown shoulder side to prevent stretching of the ply and separating of the cords. A high crown retains an increased height of the plies as they are applied, which maintains the position of the ply for forming and curing, thereby reducing distortion of the tire components and enabling the tire to have increased integrity.
It is thus desirable to provide as high a crown as possible to in turn increase the integrity of the tire as much as possible. In the prior art, a maximum crown of about 1.5 inches has been typical, but it is desirable to achieve a crown of up to about 2.50 inches. While a larger or higher crown is desirable to minimize distortion during the tire building process, it is difficult to provide a tire building drum that is able to radially contract to the needed diameter while also being able to radially expand to such a high crown distance. In addition, because expansion forces increase with a larger crown, the force that is required to achieve such a high crown is also increased, which is difficult to achieve when the drum must contract to a compact diameter.
Therefore, it is desirable to provide a tire building drum with a center mechanism that enables a high crown diameter of up to about 2.50 inches to be achieved. It is also desirable for the center mechanism to have an efficient construction that is easy to maintain.